1. Technical Field
The technical field relates to piezoelectric elements which can perform electromechanical transduction.
2. Background Art
A piezoelectric element contains an oxide ferroelectric thin film having excellent ferroelectric, piezoelectric, pyroelectric, and electro-optic properties. Well-known examples of such oxides include those having a perovskite structure with a general formula ABO3. Oxide ferroelectric thin films have been used as effective materials for a wide range of devices such as various types of sensors, actuators, and memories. Their application will probably be further expanded in the future.
Oxide ferroelectrics are typified by lead zirconate titanate (PZT) having a general formula of Pb(Zrx, Ti1-x)O3 where 0<x<1. PZT is a solid solution of PbZrO3 and PbTiO3 each having a perovskite structure. When Zr in PbZrO3 is continuously replaced by Ti, the crystal undergoes a phase transition from rhombohedral to tetragonal in the composition near Zr/Ti=53/47. This phase boundary is called the morphotropic phase boundary, and it is known that in the composition near the boundary, physical constants such as relative permittivity and piezoelectric constant become maximum. A PZT thin film has different physical constants depending on the orientation direction thereof. In other words, a PZT thin film is subjected to different distortions depending on the direction in which an electric field is applied. For example, applying an electric field in the axial direction called a polarization axis causes a large distortion. In a tetragonal PZT thin film, the polarization axis is in the c-axis direction ((001) direction), which is the longitudinal axis of the crystal lattice. Aligning the molecules in the crystal lattice in this direction (called “orientation control”) provides the PZT thin film with a high piezoelectric constant. PZT thin films are also known to have excellent linearity in the piezoelectric constant (the linearity of the amount of displacement with respect to the applied electric field).
A PZT thin film can be prepared by various methods including vapor phase growth methods such as deposition, sputtering, chemical vapor deposition (CVD), and liquid phase growth methods such as chemical solution deposition (CSD). Among these, the CSD process is a non-vacuum process and can therefore be performed with low cost. The CSD process can also prepare a precursor solution that is homogeneous on the molecular level, and is therefore easy to control the composition of the PZT thin film. Furthermore, it is possible to use spin coating, which makes the composition and thickness of the film uniform in the plane and provides high reproducibility. With spin coating, a film can be easily formed even on a large substrate.
A PZT thin film is generally formed on a Pt (111) layer, which is formed as a lower electrode layer on a silicon (Si) substrate in consideration of the consistency with the semiconductor process. Silicon, however, is too brittle to be used as a material for diaphragms in actuator devices and cracks may cause in silicon and the piezoelectric element may easily be broken. To avoid this problem, in applications requiring long-term reliability in operation, it is effective that the substrate is made of metallic materials such as stainless steel, titanium, nickel, molybdenum, or copper, or tough metallic material such as special steel or an alloy containing these metals. Examples of these application include drive elements for scanning-type projectors, and vibrating mirror elements for optical and RF switches. Among the above-mentioned metallic materials, those having a low Young's modulus, such as titanium, nickel, molybdenum, copper, and alloys containing them can be used for the diaphragms of actuator devices in order to provide a large amount of displacement.